docs(union): changelog entry + port-checklist annotations

The two kernel port checklists record the extractor configs as surveyed
at porting time; their structTypes lines are marked superseded rather
than rewritten, so the surveys stay readable as history.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -17,6 +17,8 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Fixes
 
+- Unions are now indexed in C, C++, Objective-C and Rust. A `union` declaration previously produced no symbol at all, so it never appeared in search or `codegraph_explore`, and anything attached to it disappeared with it — in Rust, every `impl SomeTrait for MyUnion` lost its edge, and the methods from that impl were left pointing at a type the graph did not contain. A `typedef union { … } Name;` in C now carries the typedef's name like `typedef struct` already did. Re-index after upgrading to pick up the new symbols.
+
 - `codegraph_explore` now concentrates its answer on the code that actually answers your question instead of spreading it across files that merely share a word with it, so more of the answer arrives in a single call. Thanks @LeDuyViet for the detailed measurements and reproduction. (#1500)
 - Files only weakly related to your question now come back as a name, symbol and line number instead of spending the answer on their source — name one of them in a follow-up `codegraph_explore` to get it back in full. (#1500)
 - A generated CRUD or protobuf layer no longer crowds out the hand-written code sitting beside it: generated files are now recognized by the `// Code generated by … DO NOT EDIT.` style banner written at the top of the file, not just by a filename that looks generated. Re-index after upgrading to pick up the new detection. (#1500)